1. Technical Field
This invention relates to driver circuits for thermal printheads employing a ribbon that generates localized heat in the ribbon in response to electrical current. The localized heat then serves to cause ink transfer to a receiving medium. Typically, the electrical signals are applied by printhead electrodes wiping across an outer layer of the ribbon which is characterized by moderate resistivity. These signals move inwardly to a layer that is highly conductive (typically an aluminum layer) with localized heating occurring in the process. The electrical circuit is completed by an electrode connected to ground which intersects the ribbon. This invention is directed to providing practical, effective, and cost-efficient circuitry to control current to the ribbon from the printhead in a manner which adjusts to electrical-circuit conditions while limiting power at the ribbon surface.
2. Background Art
The printing system to which this invention is directed and current control systems for the printhead are disclosed in U.S. Pat. No. 4,345,845 to A. E. Bohnhoff et al and U.S. Pat. No. 4,434,356 to T. P. Craig et al. The foregoing U.S. Pat. No. 4,434,356 teaches regulated constant-current circuits having each of the electrodes. The foregoing U.S. Pat. No. 4,345,845 describes a voltage source regulated in response to a voltage sensed at the ribbon at a location spaced from the printing zone. This invention also employs a regulated voltage source. The voltage is regulated in response to the level sensed at each electrode through diodes connected to each electrode. The foregoing U.S. Pat. No. 4,434,356 employs diodes connected to each electrode, but the signal is not used in a voltage-source system.
This invention also employs a voltage-divider circuit to each electrode having a resistor between the regulated voltage output and the electrode which is selected to provide at nominal conditions the same voltage drop as that across the ribbon. No directly similar design is known, but the general relationship has been understood in connection with operating bipolar transistors. The foregoing U.S. Pat. No. 4,345,845 and U.S. Pat. No. 4,420,758 to Tabata et al discloses such resistors to limit current flow. The second patent is to a range of 1/10 to 10 times the full resistance in the ribbon, while this invention is to an approximately specific value.